Browsing by Author "Wollan, D. S."
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- Dynamic nuclear polarization in samarium-doped lanthanum magnesium nitrateByvik, C. E.; Wollan, D. S. (American Physical Society, 1974-08)We report theoretical and experimental investigations of the dynamic nuclear polarization (DNP) and nuclear spin-lattice relaxation of protons in single crystals of lanthanum magnesium nitrate (LMN) weakly doped (≅0.1-1.1-mole%) with samarium ions (LMN:Sm) at 9.1 GHz and liquid-helium temperatures. Theoretical expressions for DNP and nuclear relaxation are given for a homogeneously broadened ESR line in the high-temperature limit. DNP effects due to the electron dipole-dipole reservoir (EDDR) are predicted for the case of well-resolved satellites. Our DNP experiments on LMN:Sm are in general agreement with the conventional theory, but no evidence for direct EDDR effects on DNP with well-resolved satellites is seen, despite suggestive nuclear-relaxation data. Various explanations for this disagreement are proposed. We suggest that these EDDR effects may be seen in crystals of ≅3-5-mole% LMN:Sm or in equivalent systems.
- Dynamic nuclear polarization with an inhomogeneously broadened ESR line. I. TheoryWollan, D. S. (American Physical Society, 1976-05)Theoretical investigations of the dynamic nuclear polarization (DNP) and the nuclear spin-lattice relaxation time Tn in diamagnetic dielectric crystals diluted with electron paramagnetic impurities are reported here. The steady-state DNP enhancement Ess and the DNP pump time τDNP are calculated for the solid effect (SE), the cross effect (CE), and DNP by the electron dipole-dipole reservoir (EDDR), assuming (i) a predominantly inhomogeneously broadened (IHB) ESR line with uniform spin-packet widths (for SE and CE, but not EDDR DNP), (ii) rapid nuclear spin diffusion, (iii) the electron and nuclear high-temperature limit, (iv) no phonon bottleneck, and (v) isotropic electron impurities or axially symmetric electrons whose symmetry axis (c axis) is aligned parallel to the applied magnetic field H⃗ 0 DNP results for IHB SE, which include DNP leakage factors, predict Ess comparable to the ideal enhancement in favorable cases, with Ess vs H0 curves not proportional to the ESR line-shape derivative at high microwave powers, contrary to early theories of IHB SE. The effects of electron jumps between spin packets are discussed. DNP and Tn are calculated for the cross effect for both the well-resolved and unresolved limits. We get larger SE DNP and smaller CE DNP for the unresolved CE than do Hwang and Hill, and consider CE DNP leakage factors they omitted. The inequality Tn2≤τDNP≤Tn is found for the well-resolved CE, while τDNP≈Tn is estimated for the unresolved CE, different from the general SE case where τDNP≪Tn often occurs. DNP by EDDR is extended from the pure homogeneously broadened (HB) ESR limit to the IHB case with fast spectral diffusion, as suggested by Abragam and Borghini, and is further extended from electronic isotropy to axially symmetric electrons with the c axis parallel to H⃗ 0 Various limits of Ess and τDNP are compared and contrasted for SE, CE, and EDDR DNP, to enable experimenters to differentiate these three DNP mechanisms, if possible. We propose a three-spectral-region model for wide ESR lines, which may exhibit IHB behavior in the wings of the line, HB EDDR effects near the ESR absoption peak, and CE effects in between.
- Dynamic nuclear polarization with an inhomogeneously broadened ESR line. II. ExperimentWollan, D. S. (American Physical Society, 1976-05)Results of proton dynamic nuclear polarization (DNP), nuclear spin-lattice relaxation, and ESR experiments in six YES: Er168 (Er168-doped yttrium ethyl sulphate) single crystals at 4.15 K and 9.1 GHz are reported here. This material approximates an inhomogeneously broadened (IHB) ESR line for certain crystal orientations relative to the applied magnetic field H⃗ 0. DNP peak enhancements Epeakss≈120 are obtained for crystals 1-3 having Er168 dilutions of 0.2%-0.01% (mole%). The steady-state enhancement Ess vs H0 curves are proportional to the ESR absorption derivative at low powers (≅ 0.3 mW), but change shape in the wings of the ESR line at high microwave powers (≅ 300 mW). Essentially idential Ess data are obtained for all three crystals. Extensive DNP pump time τDNP and ESR data for crystals 2-4, as well as limited data for the other crystals, are also given. There is no Ess or τDNP evidence in these experiments (i) for the traditional strict-shell-of-influence model of IHB SE (solid effect) DNP, (ii) for non-negligible CE (cross effect) DNP processes, or (iii) for decreases in Ess due to moderate spectral diffusion in an IHB ESR line, as proposed by Buishvili et al. The DNP and ESR data of crystals 1-3 have been fit by the least-squares technique to a muffin-tin model of IHB SE DNP which incorporates rapid nuclear spin diffusion. Moderate agreement with theory is obtained, especially if the data are interpreted with a three-spectral-region model for wide IHB ESR lines. These data have also been analyzed in terms of EDDR (electron dipole-dipole reservoir) DNP theory for an IHB ESR line with fast spectral diffusion throughout the line. There is qualitative and some quantitative agreement between this theory and the data. It is thus difficult to discriminate EDDR DNP theory from IHB SE DNP with the three-spectral-region model in these experiments. The only clearcut evidence for the latter is an order-of-magnitude theoretical calculation of the electron magnetic dipole-dipole contribution to the ESR intrinsic spin-packet width, which gives non-negligible spectral diffusion in the centers of the crystal 1 and 2 ESR lines, and negligible spectral diffusion in the wings of crystals 3 and (perhaps) 2. Conclusive evidence would require further experiments at Er168 dilutions lower than 0.01% and higher than 0.2%. Experiments are proposed for YES: Er168 to test further the three-spectral-region model, and to find high DNP by IHB SE with possible applications to polarized nuclear targets in other, more hydrogenous materials.